wifi: rtl8xxxu: support multiple interfaces in {add,remove}_interface()

Add a custom struct to store in vif->drv_priv with a reference to
port_num and fill it when a new interface is added. Choose a free
port_num for the newly added interface.

As we only want to support AP mode/sending beacons on port 0, only change
the beacon settings if a new interface is actually assigned to port 0.

Call set_linktype() and set_mac() with the appropriate port_num.

Signed-off-by: Martin Kaistra <martin.kaistra@linutronix.de>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20231222101442.626837-15-martin.kaistra@linutronix.de